1. Field of the Invention
The present invention relates to a method and an apparatus for drying a coated film, and in particular, relates to a method and an apparatus for drying the surface of a long and wide coated film formed on a long support by applying a coating liquid including an organic solvent thereon, when manufacturing an optical compensation sheet and the like.
2. Description of the Related Art
A liquid crystal display has an optical compensation sheet in order to improve its view angle characteristics, which is provided between a pair of polarizing plates and a liquid crystal cell as a phase difference sheet. A method of manufacturing a long optical compensation sheet includes the steps of: applying a coating liquid containing a resin for forming an orientational film on the surface of a long transparent film; and rubbing the coated film to form the orientational film. A manufacturing method further includes the steps of: applying a coating liquid containing a liquid crystalline discotic compound on the orientational film; and drying the coated film. The method of drying the coated film is disclosed in Japanese Patent Application Laid-Open No. 09-73081.
The method of drying the coating liquid containing the liquid crystalline discotic compound disclosed in Japanese Patent Application Laid-Open No. 09-73081 includes the steps of: applying the coating liquid containing the liquid crystalline discotic compound on the orientational film; and then initially drying the coating liquid in an air-conditioned atmosphere in a room to mainly vaporize an organic solvent contained in the coating liquid to dry the coating liquid, before drying the coating liquid in a regular drying apparatus.
However, the optical compensation sheet manufactured with the manufacturing method according to Japanese Patent Application Laid-Open No. 09-73081 produces two types of spots (irregularities) (A) and (B) as illustrated in FIG. 10 on the surface of the coated film 101 in an initial drying step. One is a broad spot (A) (shown by thin line), and the other one a sharp spot (B) (shown by thick line). The spots occasionally cause a problem of decreasing a yield of a product.
As a result of having analyzed these two spots (A) and (B), it was revealed that the broad spot (A) is caused by a reduced thickness of a coating liquid film 102 containing the liquid crystalline discotic compound, as is illustrated in FIG. 11. In FIG. 11, reference numeral 103 designates a long support, and reference numeral 104 designates an orientational film layer. On the other hand, it was revealed that an orientation portion 105 (deep color portion) having a sharp spot (B) formed therein showed an orientation direction 106 shifted from a regular orientation direction 107 in an orientation portion 108, as is illustrated in FIG. 12.
A general method employed as an effective measure against the spots (A) and (B) which are produced in the initial drying step includes a technique of increasing the viscosity of the coating liquid by increasing the concentration of the coating liquid or adding a thickening agent to the coating liquid; and thereby inhibiting a flow of the liquid on the surface of the coated film right after having been coated due to drying air to prevent the production of the spots. There is another method of preventing the production of the spots by using a solvent having a high boiling point. The solvent causes a leveling effect on the surface of the coated film right after having been coated, even when the liquid flowed on the surface due to the drying air.
However, a method of increasing the viscosity of the coating liquid by increasing the concentration of the coating liquid or adding the thickening agent has a problem of being incapable of performing a precise application for an ultra-thin layer in order to form a ultra-thin coated film with a high-speed application method. The method also has a disadvantage of extremely decreasing the production efficiency, because a threshold application speed (a threshold of a stable application speed) decreases along with the increase of the viscosity of the coating liquid, and the coating liquid having a higher viscosity cannot be applied at a higher speed.
On the other hand, a method of using a solvent having a high boiling point has a disadvantage of increasing a drying period of time, increasing an amount of a solvent remaining in the coated film to further increase the drying period of time, and consequently aggravating production efficiency.
With respect to such a background, the present applicant proposed a method and apparatus for drying a coated film as disclosed in Japanese Patent Application Laid-Open No. 2001-170547. The coating method and apparatus includes: providing a drying zone right directly behind an application section so as to surround the surface of a coated film to be dried on the above described running long support; and supplying a drying air so as to flow only in one direction from one end side to the other end side across the width of the above described long support in the above described drying zone. The coating method with the use of the coating apparatus can uniformly dry the coated film without changing physical properties such as a viscosity of a coating liquid or a type of a solvent. The above Japanese Patent describes that the drying method and apparatus can inhibit the production of the above described spots.